Salmonella: Encyclopedia II - Salmonella - Microbiology

Salmonella - Microbiology

Salmonella are Gram negative bacteria. In a clinical laboratory, they are usually grown isolated on blood agar, CLED agar, XLD agar or DCA agar. On blood agar, they form moist colonies about 2 to 3 mm in diameter. They do not ferment lactose.

Salmonella taxonomy is complicated. Currently (7 Dec 2005), there are two species within the genus: S. bongori (previously subspecies V) and S. enterica (formerly called S. choleraesuis), which is divided into six subspecies:

• I—enterica
• II—salamae
• IIIa—arizonae
• IIIb—diarizonae
• IV—houtenae
• V—obsolete (now designated S. bongori)
• VI—indica

There are also numerous (totalling over 2000) serovars within both species, which are found in a disparate variety of environments and which are associated with many different diseases. The vast majority of human isolates (>99.5%) are subspecies S. enterica. For the sake of simplicity, the CDC recommend that Salmonella species be referred to only by their genus and serovar:
e.g.,
Salmonella Typhi
instead of the more correct designation,
Salmonella enterica subspecies enterica serovar Typhi.

Salmonella isolates are most usually classified according to serology (Kauffman-White classification) (Salmonella subcommittee, 1900). The main division is first by the somatic O antigen, then by flagellar H antigens. H antigens are further divided into phase 1 and phase 2. The full description of a Salmonella isolate is given as (O antigens, Vi : H antigen phase 1: H antigen phase 2).

Examples:

• Salmonella Enteritidis (1,9,12:g,m)
  

(The O antigens present are 1, 9 and 12; the H antigens are g and m)

• Salmonella Typhi (9,12,Vi:d:−)

(The O antigens are 9, 12, and Vi; the H antigen is d)

In a clinical laboratory, only a small number of serovars are looked for (the remainder being rare or not clinically significant). The HPA recommend testing for the following antigens routinely:

• O antigens: 2 4 6.7 8 9 and 3.10
• phase 1 H antigens: 1 2 3 4 5 6 7
• phase 2 H antigens: a b c d E G i r

Isolates that cannot be identified using this panel are sent to the reference laboratory for identification.

Adapted from the Wikipedia article "Microbiology", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki